1. Field of the Invention
The present invention relates to a control circuit for a power supply, and more specifically, to a switching control circuit for switching mode power supplies.
2. Description of Related Art
Various power supplies have been widely used to provide regulated voltage. For the sake of safety, an off-line power supply is used to provide galvanic isolation between its primary side and secondary side. An optical-coupler and a secondary-side regulator are usually needed for regulating the output voltage of the off-line power supply. In order to reduce the device count and do without the secondary feedback circuit, a primary-side control technique has been disclosed in U.S. Pat. No. 4,302,803 “Rectifier-Converter Power Supply with Multi-Channel Flyback Inverter,” issued to Randolph D. W. Shelly, on Nov. 24, 1981. However, foregoing prior art cannot meet the standard of accurate output voltage. Further, in such a design, the power consumption at light load condition is significantly high. Accordingly, the object of the present invention is to provide a switching control circuit for precisely controlling the output voltage of a power supply at the primary side without the optical-coupler and secondary-side regulator. In addition, an off-time modulation is developed to reduce the switching frequency and save the power consumption of the power supply at the light load condition.